(1) Field of the Invention
This invention relates to a plasma display panel used in a display apparatus, and specifically to a method of producing a plasma display panel suitable for minute cell structure.
(2) Description of the Prior Art
Recently, as the demand for high-quality large-screen TVs such as high-vision TVs has increased, displays suitable for such TVs, such as Cathode Ray Tube (CRT), Liquid Crystal Display (LCD), and Plasma Display Panel (PDP), have been developed.
CRTs have been widely used as TV displays and excel in resolution and picture quality. However, the depth and weight increase as the screen size increases. Therefore, CRTs are not suitable for large screen sizes exceeding 40 inch. LCDs consume a small amount of electricity and operate on a low voltage. However, producing a large LCD screen is technically difficult, and the viewing angles of LCDs are limited.
On the other hand, it is possible to make a PDP with a large screen with a short depth, and 40-inch PDP products have already been developed.
A general PDP is composed of a front cover plate and a back cover plate to each of which electrodes are attached so that the electrodes of both cover plates face each other. A space between the front cover plate and the back cover plate is divided into a plurality of spaces by partition walls. The plurality of spaces between these partition walls are each filled with discharge gas and any of red, green, and blue fluorescent substances. The PDP with the above construction is produced first by forming the fluorescent substances in the channels between the partition walls on the back cover plate, placing the front cover plate onto the back cover plate, then charging the discharge gas. A driving circuit is used to fire the electrodes for driving.
The light-emission principle in PDP is basically the same as that in fluorescent light: a discharge lets the discharge gas emit ultraviolet light; the ultraviolet light excites fluorescent substances; and the excited fluorescent substances emit red, green, and blue lights. However, since discharge energy is not effectively converted to ultraviolet light and conversion ratio in fluorescent substance is low, it is difficult for PDPs to provide brightness as high as that of fluorescent lights.
PDPs are divided into two types: Direct Current (DC) type and Alternating Current (AC) type. The electrodes of the DC type are exposed in the discharge space, while the electrodes of the AC type are covered by a dielectrics glass layer.
The shapes of the partition walls are also different: the partition walls of the AC type are formed in stripes; the partition walls of the DC type are formed in a lattice shape. Of these, the AC type is suitable for forming a panel with a minute cell structure.
Meanwhile, as the demand for high-quality displays has increased, minute cell structures have been desired also in PDPs.
For example, in 40-inch screens conforming to the National Television System Committee (NTSC) standard, the number of pixels is 640.times.480, the cell pitch 0.43 mm.times.1.29 mm, and the square of one cell about 0.55 mm.sup.2. While in 42-inch high-vision TVs, the number of pixels is 1,920.times.1,125, cell pitch 0.15 mm.times.0.48 mm, and square of one cell 0.072 mm.sup.2.
To bring such PDPs with minute cell structures into practical use, the light emission efficiency should be increased. As a result, studies for improving fluorescent substances, for example, are under way for this purpose.
However, the problems shown below are seen in forming fluorescent substance layers.
As shown in FIG. 1, a popular conventional method of forming a fluorescent substance layer uses the screen printing method in which fluorescent substance pastes are supplied to depression parts between the partition walls and they are baked. However, it is difficult to apply the screen printing method to PDPs with minute cell structures.
When the cell pitch is in a range of 0.1-0.15 mm, the width of each space between the partition walls becomes very narrow, namely, in the range of 0.08-0.1 mm. Fluorescent substance inks used in the screen printing have high viscosity (generally, several hundreds of thousands centi-poise (cP)). It is difficult to pour such a high-viscosity fluorescent substance ink into a narrow channel between the partition walls accurately and at high speed.
To acquire high-light-emission PDPs, it is desirable to construct the PDPs so that the fluorescent substance layer is formed not only on the surface of the back plate but on the sides of the partition walls and that discharge spaces are secured between the partition walls. To fulfill the above construction in the screen printing method, for example, an appropriate amount of fluorescent substance paste should be applied onto the surface of the back plate and onto the sides of the partition walls by controlling the viscosity of the fluorescent substance paste. However, it is difficult to set the viscosity of the fluorescent substance paste to an appropriate level. It is also difficult to apply the fluorescent substance paste onto the sides of the partition walls.
There are other methods of forming the fluorescent substance layer than the screen printing method, such as the photoresist film method and the ink jet method.
Japanese Laid-Open Patent No. 6-273925 describes the photoresist film method. According to the description, a ultraviolet ray photosensitive resin film containing fluorescent substances with various colors are embedded in the channels between the partition walls, only the film parts which are to be the fluorescent substance layers of desired colors are exposed, and the rest of the film is swept by a liquid. It is possible with this method to embed the film into channels between the partition walls accurately even if the cell pitch is narrow. However, the production procedure of this method is complex since the film embedding and sweeping should be repeated for each of the three colors. Moreover, the method often allows the colors to mix with each other. The method also has a problem of cost since it is difficult to collect the swept fluorescent substances though the fluorescent substances are relatively expensive.
Japanese Laid-Open Patents No. 53-79371 and No. 8-162019 disclose the ink jet method. According to the disclosure, an ink, containing fluorescent substances and organic binders, is spouted out of running nozzles onto the surface of an insulating substrate when put pressure so that a desired pattern is drawn on the surface. This method also enables an application of the ink onto surfaces of the narrow channels between the partition walls.
However, when the partition walls are formed in stripes, it is difficult for the method to form a layer of the applied ink with a constant layer thickness since the ink is applied intermittently in the form of liquid drops. The method also has the same problem as the photoresist film method, that is, it is difficult to apply the fluorescent substance paste onto the sides of the partition walls.
Meanwhile, there is another known method for PDPs in which reflection layers are first formed inside the depression parts between the partition walls, then fluorescent substance layers are formed on the reflection layers (e.g. Japanese Laid-Open Patent No. 4-332430).
The screen printing method may also be used to apply a paste containing a reflection material to the parts between the partition walls to generate the reflection layers. However, forming of the reflection layers with the screen printing method has the same problems as that of the fluorescent substance layers, that is, it is difficult to apply the reflection material paste to minute cell structures and difficult to apply the reflection material paste onto the sides of the partition walls.
Another problem in forming the fluorescent substance layers or the reflection layers is that the fluorescent substances or the reflection materials often stick to the top of the partition walls. When this happens, the adhesion between the top of the partition walls and the front cover plate may be weakened when they are bonded with each other.
There is another problem concerning forming of electrodes. In conventional PDPS, the width of display electrodes or address electrodes is 130-150 .mu.m. These electrodes are generally formed with the screen printing method. However, in case of the high-vision TVs, the width should be around 70 .mu.m considering the number of pixels. In case of a higher-vision 20-inch SXGA (Super eXtended Graphics Array) (the number of pixels is 1,280.times.1,024), the width should be around 50 .mu.m. It is difficult to form electrodes with such widths with the screen printing method.